Tension leg platforms are shown in U.S. Pat. Nos. 2,399,611, 3,154,038, 3,648,638, 3,780,685, and 3,934,528. Such tension leg platforms generally comprise a partially submerged floatable platform connected to suitable anchor means on the sea floor by tension members. Such tension members or tension legs generally comprise wire cable, chain, and pipe. Tension is applied to the tension legs by selected ballasting of the platform so that a buoyant upwardly directed force is exerted on the tension legs.
Prior proposed vertical members extending between a floatable platform and anchor means at the sea bottom have included neutrally buoyant vertical anchor chains comprising buoyant sections interconnected by interlocked eyes and laterally extending neutrally buoyant mooring lines, U.S. Pat. No. 2,939,291. Tension legs, including conduit members, are shown in U.S. Pat. Nos. 3,355,899 and 3,154,039.
Vertical riser or casing systems not used as tension legs include riser sections having swivel joints therebetween and maintained under tension by a float located below the surface of the water, U.S. Pat. No. 3,196,958; gimbal joints between riser sections, U.S. Pat. No. 3,523,578; the riser held in upright position by a buoyant sphere, U.S. Pat. No. 3,605,668; and a casing provided with longitudinally spaced buoyancy pods separate from the casing, U.S. Pat. No. 3,017,934.
Prior proposed tension leg platforms have included the capability of carrying anchor means in association with the platform as it is moved above a sea floor site. Such anchor means are then lowered by suitable means to the sea floor and ballasted to provide gravity, deadweight, or other suitable anchor holding forces. Prior proposed methods of ballasting an anchor means on the sea floor have included lowering an anchor shell by drill pipe, and using the drill pipe for flow of ballast material to the anchor shell, U.S. Pat. No. 3,934,528. In other prior methods of ballasting such an anchor, a separate line for conducting ballast material to the anchor shell was employed, U.S. Pat. No. 2,399,611. Such separate lines, called tremie lines, required special handling and special equipment.